[Tutor] Python oddity

[Jeff Younker]

So the problem is that when I change bb, aa also changes even
though I don't want it to.  Is this supposed to happen?

   Yes

Names are handles for objects.  Assignment binds
a name to an object.   The same object can be
bound simultaneously to many names.

Python distinguishes between equality and identity, so
there are two comparison operators:  '==' and 'is'.    Objects
are equal if they have the same value (==) but they are
identical only if they refer to the same object (is).

The constructor expressions [], {}, and () produce
new objects.

 >>> [1, 2] == [1, 2] # same value
True
 >>> [1, 2] is [1, 2] # but different lists
False

 >>> a = [1, 2] # this is assigned to a new list
 >>> b = [1, 2] # this is also assigned to a new list
 >>> a == b # the lists have the same value
True
 >>> a is b # but they are not the same list
False

 >>> a = [1, 2] # assign to a new list
 >>> b = a # attach the list in a to a new name
 >>> a == b # they have the same value
True
 >>> a is b # because they are the same list
True

The list(FOO) function copies an list.

 >>> a = [1, 2] # new list
 >>> b = list(a) # make a copy of the list in b
 >>> a == b # the original and copy are equal
True
 >>> a is b # but they are not the same list
False



- Jeff Younker - jeff at drinktomi.com -



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